The present invention relates to a vehicle speed control apparatus which automatically maintains a vehicle speed at a desired vehicle speed without adjustments by the driver. More particularly, the present invention relates to improvements for reducing an initial decrease in the actual vehicle speed after the desired vehicle speed is set in a memory.
FIG. 12 shows a diagram of actual vehicle speed after the commencement of the acceleration of the vehicle having a first conventional vehicle speed control apparatus. In FIG. 12, a value `Vset` of a desired vehicle speed is set in the memory of the vehicle speed control apparatus at an initial time `ti` of automatic vehicle speed control. This conventional vehicle speed control apparatus can maintain the value `Vset` of the desired vehicle speed after a first time period `T.sub.1 ` has passed.
However, a vehicle with this speed control apparatus suffers a long initial decrease in the actual vehicle speed, after the value `Vset` of the desired vehicle speed is set in the memory.
When the value `Vset` of the desired vehicle speed is set, the driver stops manual operation on the accelerator pedal which adjusts the degree of opening of a throttle valve and an actuator of a conventional vehicle speed control apparatus controls the degree of the opening of the throttle valve. However, the conventional actuator has a time lag in its operation. As a result, the degree of opening of the throttle valve is smaller than desired and the valve of the actual vehicle speed will be too small. Thus, the conventional vehicle speed control apparatus has a long initial decrease in the actual vehicle speed caused by the time lag in the operation of the actuator.
A second conventional vehicle speed control apparatus includes a quick response means which can output a maximum value signal when the desired speed is set in a memory. As a result, the actuator of this vehicle speed control apparatus will quickly open a throttle valve to a maximum amount, and the vehicle speed will quickly increase. This vehicle speed control apparatus therefore has an initial decrease in the deisred vehicle speed for a time period `T.sub.2 ` as shown in FIG. 13 which is shorter than the initial time `T.sub.1 ` for decrease in vehicle speed in the first conventional control apparatus.
However, when the quick response means of this second conventional vehicle speed control apparatus opens the throttle valve to the maximum amount during the time period `T.sub.2 ` the actual vehicle speed of the vehicle increases to more than the value `Vset` of the desired vehicle speed, after the time period `T.sub.2 ` has passed.
The actual vehicle speed then decreases to the value `Vset` of the desired vehicle speed after a time period `T.sub.3 ` has passed.